One Bad Transporter and Cystic Fibrosis

1. Cell membranes must be very selective to keep conditions inside the cell favorable for survival.



2. Sometimes there is a defect in the CFTR transporter protein.
   
   
   1. Not enough chloride and water cross; mucus becomes thick.
   
   
   
   2. Cystic fibrosis is a serious disease that can result from this deficiency.

5.1   Membrane Structure and Function

1. Revisiting the Lipid Bilayer
   
   
   1. The "fluid" portion of the cell membrane is made of phospholipids.
      
      
      1. A phospholipid molecule is composed of a hydrophilic head and two hydrophobic tails.
      
      
      
      2. If phospholipid molecules are surrounded by water, their hydrophobic fatty acid tails cluster and a bilayer results; hydrophilic heads are at the outer faces of a two-layer sheet.
   
   
   2. Bilayers of phospholipids are the structural foundation for all cell membranes.


2. What Is the Fluid Mosaic Model?
   
   
   1. Cell membranes are of mixed composition including the following:
      
      
      1. Phospholipids differ in their hydrophilic heads and the length and saturation of their fatty acid tails.
      
      
      
      2. Glycolipids have sugar monomers attached at the head end.
      
      
      
      3. Cholesterol is abundant in animal membranes; phytosterols occur in plants.
   
   
   2. Within a bilayer, phospholipids show quite a bit of movement; they diffuse sideways, spin, flex their tails to prevent close packing and promote fluidity, which also results from short-tailed lipids and unsaturated tails (kink at double bonds).
   
   
   
   3. The arrangement of molecules on one side of the membrane differs from that on the other side (asymmetrical).

5.2   A Gallery of Membrane Proteins

1. Where Are the Proteins Positioned?
   
   
   1. Integral proteins span the lipid bilayer, with their hydrophilic domains extending past both surfaces.
   
   
   
   2. Peripheral proteins are positioned at the surface of the membrane.


2. What Are Their Functions?
   
   
   1. Adhesion proteins are glycoproteins that help cells stay connected to one another in a tissue.
   
   
   
   2. Communication proteins form channels that match up across the plasma membranes of two cells, letting signals to flow between their cytoplasms.
   
   
   
   3. Receptor proteins have binding sites for hormones (and like substances) that can trigger changes in cell action, as in growth processes.
   
   
   
   4. Recognition proteins identify the cell as a certain type, help guide cells into becoming issues, and function in cell-to-cell recognition and coordination.
   
   
   
   5. Transport proteins passively allow water-soluble substances to move through their interior, which opens on both sides of the bilayer.

5.3   Focus on Science: Do Membrane Proteins Stay Put?

5.4   Think Diffusion

1. All cell membranes show selective permeability, that is, some substances can cross, others cannot.
   
   
   1. Gases and small electrically-neutral molecules can readily cross the lipid bilayer.
   
   
   
   2. Glucose and other large, polar molecules cannot pass through the bilayer directly but must rely on passage through the interior of transport proteins.


2. What Is a Concentration Gradient?
   
   
   1. Concentration refers to the number of molecules (or ions) of a substance in a given volume of fluid.
   
   
   
   2. The thermal energy of the molecules drives the movement of molecules.
      
      
      1. Molecules constantly collide and tend to move down a concentration gradient (high to low).
      
      
      
      2. The net movement of like molecules down a concentration gradient is called diffusion; each substance diffuses independently of other substances present as illustrated by dye molecules in water.


3. What Determines Diffusion Rates?
   
   
   1. Several factors influence the rate and direction of diffusion: concentration differences, temperature (higher = faster), molecular size (smaller = faster), electric gradients (a difference in charge), and pressure gradients.
   
   
   
   2. When gradients no longer exist, there is no net movement (dynamic equilibrium).

5.5   Types of Crossing Mechanisms

1. In passive transport, material passes through the interior of transport proteins without an energy boost; this is also known as "facilitated" diffusion.



2. In active transport, proteins become activated to move a solute against its concentration gradient.



3. Substances move in bulk across the cell membrane by exocytosis and endocytosis.

5.6   How Do the Transporters Work?

1. When water-soluble molecules bind to transport proteins, they trigger changes in shape that "ease" the solute through the protein and hence through the membrane.



2. Passive Transport
   
   
   1. A carrier protein that functions in passive transport (also called "facilitated diffusion") tends to move molecules to the side of the membrane where they are less concentrated.
   
   
   
   2. Passive transport will continue until solute concentrations are equal on both sides of the membrane or other factors intervene.


3. Active Transport
   
   
   1. To move ions and large molecules across a membrane against a concentration gradient, special proteins are induced to change shape (in a series), but only with an energy boost from ATP.
   
   
   
   2. An example of active transport is the sodium-potassium pump of the neuron membrane, and the calcium pump of most cells.

5.7   Which Way Will Water Move?

1. Osmosis
   
   
   1. Bulk flow is the tendency of different substances in a fluid to move together in the same direction due to a pressure gradient (as in animal circulatory systems).
   
   
   
   2. Osmosis is the passive movement of water across a differentially permeable membrane in response to solute concentration gradients, pressure gradients, or both.
   
   
   
   3. For example, if a bag containing a sugar solution is placed in pure water, the water will diffuse inward (higher to lower).


2. Effects of Tonicity
   
   
   1. Tonicity denotes the relative concentration of solutes in two fluids--extracellular fluid and cytoplasmic fluid, for example.
   
   
   
   2. Three conditions are possible:
      
      
      1. An isotonic fluid has the same concentration of solutes as the fluid in the cell; immersion in it causes no net movement of water.
      
      
      
      2. A hypotonic fluid has a lower concentration of solutes than the fluid in the cell; cells immersed in it may swell.
      
      
      
      3. A hypertonic fluid has a greater concentration of solutes than the fluid in the cell; cells in it may shrivel.
   
   
   3. Cells either are dependent on relatively constant (isotonic) environments or are adapted to hypotonic and hypertonic ones.


3. Effects of Fluid Pressure
   
   
   1. Hydrostatic pressure is a force directed against a membrane by a fluid; the greater the solute concentration, the greater will be the hydrostatic pressure it exerts.
   
   
   
   2. This force is countered by osmotic pressure, which prevents any further increase in the volume of the solution.
   
   
   
   3. When plants lose water, there is a shrinkage of the cytoplasm called plasmolysis.

5.8   Membrane Traffic To and From the Cell Surface

1. Exocytosis and Endocytosis
   
   
   1. In exocytosis, a cytoplasmic vesicle moves substances from cytoplasm to plasma membrane where the membranes of the vesicle and cell fuse.
   
   
   
   2. Endocytosis encloses particles in small portions of plasma membrane to form vesicles that then move into the cytoplasm.
      
      
      1. In receptor-mediated endocytosis, specific molecules are brought into the cell by specialized regions of the plasma membranes that form coated pits which sink into the cytoplasm.
      
      
      
      2. In bulk-phase endocytosis, a vesicle forms around a small volume of extracellular fluid without regard to what substances might be dissolved in it.
      
      
      
      3. Phagocytosis, is an active form of endocytosis by which a cell engulfs microorganisms, particles, or other debris; this is seen in protistans and white blood cells.


2. Membrane Cycling
   
   
   1. Even as exocytosis and endocytosis disrupt the plasma membrane, the rates are such that the plasma membrane is continually replaced.
   
   
   
   2. For example in neurotransmitter release, an episode of exocytosis was immediately followed by counterbalancing endocytosis.

Return to Top

© Copyright 2004 Thomson. All rights reserved.